System and method for stand-off monitoring of nuclear reactors using neutron detection

ABSTRACT

A system for monitoring fissile material contents inside of a nuclear reactor can include at least a first neutron detector positioned outside a radiation shield and configured to detect a plurality of neutrons originating from the reactor core and having passed through the radiation shield, and configured to generate a first output signal, and a controller communicably linked to the first neutron detector to receive the first output signal and a power output of the nuclear reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation Ser. No. 15/712,898 filed Sep. 22,2017 and entitled System And Method For Stand-Off Monitoring Of NuclearReactors Using Neutron Detection, which claims the benefit U.S.Provisional Application No. 62/512,828 filed May 31, 2017 and entitledSystem And Method For Stand-Off Monitoring Of Nuclear Reactors UsingNeutron Detection, the entirety of these applications being incorporatedherein by reference.

FIELD

The present subject matter of the teachings described herein relatesgenerally to systems for monitoring ionizing radiation and methods ofusing such systems.

BACKGROUND

Nuclear reactor safeguards measures are used to verify that nuclearmaterial is not diverted from peaceful uses. Scenarios for diversion ofnuclear material from peaceful uses can take a number of forms, such asundeclared changes in the rate of plutonium production within a reactor,undeclared reductions in the level of irradiation of fuel to facilitatelater removal of fissile material, or the actual diversion of fissilematerial from the reactor. Safeguards monitoring systems are currentlyin place at some of the world's power reactors, at research reactorsworldwide, and at other nuclear facilities that fall under theInternational Non-Proliferation Treaty. Existing safeguard systems tendto be indirect means that do not involve the direct measurement of thefissile isotopic content of the reactor, but instead consist primarilyof semi-annual or annual inspections of coded tags and seals placed onfuel assemblies, and measures such as video surveillance of spent fuelcooling ponds and the like. Direct measurements are typically madeoff-line, before or after fuel are introduced into the reactor.Real-time, online quantitative measurements of reactor core power andisotopic composition have been demonstrated in more recent times withrelatively large and relatively expensive anti-neutrino detectors. Thistechnique is generally based upon variations in detectable antineutrinoyield from differing isotopes. However, count rates of anti-neutrinodetectors tend to be relatively low, and may have a relatively very highbackground noise level.

US patent publication no. 2016/0195622 relates to an apparatus fordetecting the presence of a nuclear reactor by the detection ofantineutrinos from the reactor that can include a radioactive samplehaving a measurable nuclear activity level and a decay rate capable ofchanging in response to the presence of antineutrinos, and a detectorassociated with the radioactive sample. The detector may be responsiveto at least one of a particle or radiation formed by decay of theradioactive sample. A processor associated with the detector cancorrelate rate of decay of the radioactive sample to a flux of theantineutrinos to detect the reactor.

A publication, J. S. Beaumont et al., Nature Communications, vol. 6,article no. 9592, describes the use of neutron and gamma radiationimaging outside of reactor shielding for the purpose of diagnosing theneutron distribution in a nuclear reactor, for ensuring safe andefficient burnup of the reactor fuel. In this publication, no method isdeveloped for monitoring reactor core fuel inventory for the purposes ofnuclear material safeguards.

SUMMARY

This summary is intended to introduce the reader to the more detaileddescription that follows and not to limit or define any claimed or asyet unclaimed invention. One or more inventions may reside in anycombination or sub-combination of the elements or process stepsdisclosed in any part of this document including its claims and figures.

In accordance with one aspect of the teachings described herein, asystem for monitoring at stand-off distance a nuclear reactor having areactor core containing nuclear fuel material and a radiation shield,may include at least a first neutron detector positioned outside theradiation shield and configured to detect a plurality of neutronsoriginating from the reactor core and having passed through theradiation shield, and configured to generate a first output signal. Thesystem can also include a controller communicably linked to the firstneutron detector to receive the first output signal and a power outputof the nuclear reactor. The controller may be configured to determineaberrant changes in isotopic composition of the nuclear fuel in thereactor core, based on deviations from accepted baseline behavior in theoutput signal and the power output.

The system may optionally include a second neutron detector positionedoutside the radiation shield and spaced apart from the first neutrondetector. The second neutron detector may be configured to detect theplurality of neutrons originating from the reactor core and havingpassed through the radiation shield, and may be configured to generate asecond output signal. The controller may be communicably linked to thesecond neutron detector to receive the second output signal anddetermine an isotopic concentration of the nuclear fuel material in thereactor core based on the first output signal, the second output signaland the power output.

The controller may be operable to compare the first output signal to apre-determined, expected value of neutron flux per unit reactor powerand generate an alert if the first output signal differs from theexpected value of neutron flux per unit reactor power.

The expected neutron flux per unit reactor power may include at leastone of a simulated neutron flux per unit reactor power and a base lineneutron flux per unit reactor power for the reactor. The base lineneutron flux per unit reactor power may be an empirically generatedvalue of neutron flux per unit reactor power obtained by monitoring thereactor with the system during a calibration session when the reactor isoperating under known conditions and storing the measured neutron fluxper unit reactor power in the controller.

The first neutron detector and second neutron detector may be large-areathermal neutron detectors or fast neutron detectors, or a combination ofeach.

The system may include a first moderator surrounding the first neutrondetector, between the detector and the reactor radiation shield. Thefirst moderator may be configured to convert at least some of theplurality of neutrons from fast neutrons to thermal neutrons before theplurality of neutrons reaches the first neutron detector. The moderatormay be formed from high density polyethylene, and optionally may have athickness of between about 0.5 inches and about 3 inches, or about 1inch, in a direction that the plurality of neutrons pass through thefirst moderator. The moderator also may optionally employ materialscontaining hydrogen, deuterium, beryllium, and carbon.

In accordance with another broad aspect of the teachings describedherein, a method of monitoring the operating conditions of a nuclearreactor having a reactor core containing nuclear fuel material and aradiation shield, may include the steps of:

a) positioning at least a first neutron detector at a first stand-offlocation relative to the reactor, wherein the first neutron detector isoutside the radiation shield;

b) detecting a plurality of neutrons that originated within the reactorcore and have passed through the radiation shield using the firstneutron detector and transmitting a first output signal to a systemcontroller;

c) monitoring the reactor power output with system controller; and

d) comparing the first output signal to an expected neutron flux perunit reactor power for the reactor and generating a correspondingcontroller output signal.

Step a) may include positioning a second neutron detector at a secondstand-off location relative to the reactor. The second neutron detectormay be outside the radiation shield and may be spaced apart from thefirst neutron detector. The method may also include detecting aplurality of neutrons that originated within the reactor core and havepassed through the radiation shield, using the second neutron detectorand transmitting a second output signal.

Step d) may include comparing the first output signal with the secondoutput signal to identify differences between the first output signaland the second output signal.

The method may include comparing at least one of the first output signaland the second output signal to a pre-determined, expected neutron fluxvalue per unit reactor power and generating an alert if the first outputsignal differs from the expected value of neutron flux per unit reactorpower

The expected neutron flux may include at least one of a calculatedneutron flux and an empirically measured base line neutron flux for thereactor.

The base line neutron flux may be an empirically generated neutron fluxvalue obtained by monitoring the reactor with the system during acalibration session when the reactor is operating under known conditionsand storing the measured neutron flux in the controller.

The method may include moderating the plurality of neutrons thatoriginated within the reactor core and have passed through the radiationshield using a moderator prior to the plurality of neutrons reaching thefirst neutron detector, whereby at least a portion of the neutronsreaching the first neutron detector are thermal neutrons.

DRAWINGS

The drawings included herewith are for illustrating various examples ofarticles, methods, and apparatuses of the teaching of the presentspecification and are not intended to limit the scope of what is taughtin any way.

In the drawings:

FIG. 1A is a schematic representation of one example of a monitoringsystem positioned around a nuclear reactor;

FIG. 1B is another representation of the monitoring system and reactorof FIG. 1A;

FIG. 2 is a schematic representation of one example of a neutrondetector;

FIG. 3 another representation of the neutron detector of FIG. 2 ;

FIG. 4 is a plot showing a correlation between detector count rate of aneutron detector and thermal reactor power in accordance with at leastone embodiment;

FIG. 4A is a plot showing a correlation between detector count rate of aneutron detector and thermal reactor power in accordance with at leastone embodiment;

FIG. 4B is a plot showing a correlation between detector count rate of aneutron detector and thermal reactor power in accordance with the sameembodiment as FIG. 4A, but at a different location.

FIG. 5 is a plot showing measured and simulated detector count inaccordance with at least one embodiment of a monitoring system;

FIG. 6 is a flowchart of a method for stand-off monitoring of a nuclearreactor based on neutron detection in accordance with at least oneembodiment;

FIG. 7 is a plot of detector count rate vs moderator material thicknessfor a given neutron detector;

FIG. 8 is a top-down view of a schematic representation of stage 2 of asimulation model. The circles show the detector locations.

FIGS. 9 and 10 are plots showing calculated neutron energy spectra onthe south side of ZED-2, and the east side of ZED-2, respectively.Simulated LEU core data is shown using a thin line, and simulated NUcore data is shown using the thicker line.

FIG. 11 is a plot showing the integral sums of energy spectra regionsfrom FIGS. 9 and 10 . Black columns correspond to East Side of thereactor for a LEU core, columns with dense stripes is for the South Sideof the reactor for a LEU core, columns with moderate density stripes isfor the East Side of the reactor for a NU core, and columns with sparsestripes is for the South Side of the reactor for a NU core;

FIG. 12 is a plot showing B10+ detector count rate (solid curve) andaverage reactor power (dotted curve) versus time at Location A;

FIG. 13 is a plot showing a ratio of reactor power to B10+ detectorcount rate versus time at Location A, using the data shown in FIG. 12 ;

FIG. 14 is a plot showing a B10+ detector count rate at location B, andsimultaneous BCS detector count rate at Location A, versus time;

FIG. 15 is plot showing the B10+ detector count rate at location Bversus the simultaneous BCS detector count rate at Location A; and

FIG. 16 is a plot showing an example of how the B10+ detector count ratevaries with online refueling activities, in which (1) fuel rod flaskparks on top of reactor, (2) fuel rod taken up by flask, rapidlyemitting decay products, (3) flask moves away from top of reactor for abreak, (4), flask moves back onto top of reactor, (5) fuel rod taken upby flask, followed by rapid decay, (6) flask moves away from top ofreactor.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover processes or apparatuses that differ from those describedbelow. The claimed inventions are not limited to apparatuses orprocesses having all of the features of any one apparatus or processdescribed below or to features common to multiple or all of theapparatuses described below. It is possible that an apparatus or processdescribed below is not an embodiment of any claimed invention. Anyinvention disclosed in an apparatus or process described below that isnot claimed in this document may be the subject matter of anotherprotective instrument, for example, a continuing patent application, andthe applicants, inventors or owners do not intend to abandon, disclaimor dedicate to the public any such invention by its disclosure in thisdocument.

Reactor safeguards regimes, for detecting undeclared nuclear materialand for monitoring nuclear reactors, can be generally intended to detectillicit or suspicious uses of reactor facilities. Examples of illicituse could include unauthorized changes in the rate of plutoniumproduction within a reactor, a reduction in the level of irradiation offuel to facilitate later removal of fissile material, or the actualdiversion of fissile material from the reactor. Some versions ofsafeguard monitoring systems are currently in place at about half of theworld's power reactors, and at hundreds of research reactors worldwide.These are largely safeguarded by means that do not involve the ongoingdirect measurement of the fissile isotopic content of the reactor or thereactor core power, but instead consist of semi-annual or annualinspections of coded tags and seals placed on fuel assemblies, andmeasures such as video surveillance of spent fuel cooling ponds.

Nuclear reactor safeguard measures may be used to help verify thatnuclear material is not diverted from peaceful uses. Scenarios fordiversion of nuclear material from peaceful uses can take a number offorms, including undeclared changes in the rate of plutonium productionwithin a reactor, undeclared reductions in the level of irradiation offuel to facilitate later removal of fissile material, or the actualdiversion of fissile material from the reactor. Safeguards monitoringsystems are currently in place at some power reactors and researchreactors worldwide. These reactors are largely safeguarded by indirectmeans that do not involve the direct measurement of the fissile isotopiccontent of the reactor, but instead consist primarily of semi-annual orannual inspections of coded tags and seals placed on fuel assemblies,and measures such as video surveillance of spent fuel cooling ponds.Direct measurements may be typically made off-line, before or after fuelis introduced into the reactor.

There are however, some systems that attempt to provide real-timequantitative measurements of reactor core power and isotopic compositionmay help facilitate enhanced monitoring of nuclear reactors. One suchsystem utilizes anti-neutrino detectors. Anti-neutrinos can be producedin large quantities as by products within the nuclear reactor core offission reactions. The probability of anti-neutrinos interacting withmaterial is relatively low, which means that substantially all of theanti-neutrinos created can the stream out of the nuclear reactor core.The fission yield of the reactor is generally proportional to thereactor power, and therefore so is the anti-neutrino yield.Anti-neutrino yield can also vary with fissioning isotopes, such thatanti-neutrino detectors may be designed to follow changes in reactorcore isotopic composition through the reactor fuel cycle.

However, because anti-neutrinos are weakly-interacting materials,anti-neutrino detectors typically require a relatively large detectionvolume, typically on the scale of a few cubic meters. The detectorstypically weigh on the scale of a few tonnes. Their antineutrinodetection rates tend to be relatively low, and the antineutrino signalis typically discriminated against a large amount background noiseinduced by other particles with relatively higher interactionprobabilities. Sophisticated veto and discrimination strategies andsignal processing systems are required for extracting the trueantineutrino signal, adding to the complexity of the overall detectorsystem. Detectors of this size and complexity may not be suitable forsome reactor monitoring systems, and/or may be difficult to integrateinto existing reactor facilities and the like. In some environments, therelatively low signal-to-noise ratio of these systems can reduce theoverall sensitivity of the monitoring systems. Constructing andinstalling anti-neutrino monitoring systems may also be relativelycostly.

In contrast to existing monitoring systems, a new nuclear monitoringsystem has been created that may help facilitate real-time, ongoingdirect measurement of the fissile isotopic content of the reactor or thereactor core power using neutron detectors that are positioned insuitable locations around a nuclear reactor, rather than anti-neutrinodetectors. Specifically, the neutron detectors are positioned outsidethe reactor core and outside the standard radiation shielding materialthat surrounds a reactor core, and are an example of a so calledstand-off monitoring system in which the detectors are positionedoutside the stand-off distance surrounding the nuclear reactor. For thepurposes of this description, a standoff distance can be any suitabledistance from a nuclear reactor that is outside the radiation shieldingof the reactor core, and may be between 10 m and about 1000 m or morefrom the reactor core, and may be between about 20 m and about 500 m,and between about 50 m and about 200 m or optionally within 100 moutside the boundary of the radiation shielding of the reactor core. Insome configurations, the neutron detectors may be positioned outside theradiation shielding layer and within the surrounding reactor building,and in other configurations the neutron detectors may be located outsidethe reactor building, and optionally may be in a separate building,trailer, enclosure or other suitable location.

While it is understood that anti-neutrino particles escape from thereactor core and are present outside the radiation shielding layer of anuclear reactor, it is not obvious that neutrons would be present insufficient quantities to make meaningful measurements of the reactorcondition(s) outside the radiation shielding layer of a nuclear reactor,in part because the radiation shielding layer in a fission-based nuclearreactor is intended to prevent the escape of neutrons from the reactorcore.

Prior to developing the new monitoring systems described herein, it wasunknown if positioning neutron detectors outside the stand-off perimeterto monitor the fissile isotopic content of the reactor or the reactorcore power would allow for the creation of an acceptable monitoringsystem for a variety of reasons, including that It was not clear apriori that there would be sufficient neutron flux escaping nuclearreactor shielding that could be detected at stand-off distances from thereactor, it was not clear a priori that the neutron detection signalacquired outside of nuclear reactor shielding would be sensitive to thereactor core's isotopic fuel composition, and it was not clear a priorihow environmental influences on neutron detection rate might beovercome. However, despite these apparent challenges, it wassurprisingly discovered that neutron detectors can be utilized assensors in a stand-off reactor monitoring system, and that the signalsreceived from the neutron detectors could be used to detect differencesin the fissile isotopic content of the reactor or the reactor core powerover time.

The inventors discovered that neutron detection at stand-off distancesusing an array of large-area neutron detectors at various locationswithin and around a reactor facility could potentially provide a viable,economical, and relatively more compact alternative to anti-neutrinodetectors. As described in more detail herein, neutron detection may beable provide a system with an acceptable signal to background, and theresults of detection may be sensitive to the differences inisotopic-dependent properties of the fuel within the reactor includingthe energy released per fission, and the cross-section for fission.Isotopic properties for U-235, U-238 and Pu-239 are summarized in Table1.

TABLE 1 PROPERTY U-235 U-238 Pu-239 Pu-241 NUMBER OF 1.921 ± 0.019   2.381 ± 0.020    1.451 ± 0.021    1.831 ± 0.019    ANTINEUTRINOS ABOVE1.8 MeV PER FISSION [10] THERMAL NEUTRON  399.4 ± 1.1 barns 0.000002barns  512.8 ± 2.0 barns  693.1 ± 6.2 barns INDUCED FISSION CROSSSECTION AT 433 K [7] THERMAL ENERGY 201.7 ± 0.6 MeV 205.0 ± 0.9 MeV210.0 ± 0.9 MeV 212.4 ± 1.0 MeV PER FISSION [10]

Comparing the properties of the neutrons measured using the system(s)described herein, in view of these isotopic-dependent properties andoptionally with reference to other reactor parameters including poweroutput, may help facilitate detection of changes in isotopic compositionof the fuel in a reactor core, while monitoring the power and escapingneutron output of the reactor. One potentially advantageous economicalaspect of the systems described herein that use neutron detectors forthe purpose of reactor safeguard measurements, may be that an array ofneutron detectors can be deployed at different locations around areactor to obtain readings of the neutron flux per unit reactor power ateach location. The system can then monitor each of these discretereadings over time. If an event is recorded on one detector (for examplea spike in neutron flux per unit reactor power), the readings of otherdetectors taken at the same time can be queried for evidence of similarevent. If the same phenomenon is detected on multiple detectors, it maysuggest that the phenomenon originated from within the reactor. Incontrast, if an event is detected at one detector but not others, it maysuggest a background or environmental occurrence that is localized toone of the detectors and is not necessarily indicative of a change inthe reactor conditions.

Optionally, the readings from the multiple neutron detectors may becoordinated, e.g. time-stamped, so as to help facilitate discriminationagainst either inadvertent or malicious interferences that might causevariations in individual neutron detection rates.

As compared with anti-neutrino detectors that could be used for reactormonitoring, neutron detectors may tend to be relatively smaller. Usingrelatively smaller, lightweight detectors may help facilitatetransportation and installation of the neutron detectors. This may behelpful if the system is configured as a portable system that can betransported, and set-up around, multiple different nuclear reactors.Providing relatively small neutron detectors may also help facilitateplacing the neutron detectors in desired locations around a givennuclear reactor, and may make the system more flexible in terms ofphysical arrangement than a comparable anti-neutrino monitoring system.

As compared to anti-neutrinos, neutrons interact with materials far morereadily and may be relatively easier to detect. That is, the event ratefor anti-neutrino detectors may be one or more orders of magnitude lowerthan the event rate for a neutron detector suitable for use with thesystems described herein. As such, the inventors have discovered that arelatively lower flux of neutrons escaping the shielded reactor core (ascompared to the flux of escaping anti-neutrinos) may be detected inquantities that are sufficient for use with the systems and methodsdescribed herein.

Some of the teachings described herein are based on the understandingthat the number of neutrons detected, n_(det), is proportional to thepopulation of neutrons n_(pop) in the reactor core,

$\begin{matrix}{n_{\det} \propto n_{pop} \propto {\frac{\left\langle \phi \right\rangle}{\left\langle v \right\rangle}V}} & (1)\end{matrix}$where <ϕ> is the average neutron flux in the reactor core, <v> is theaverage speed of the neutrons in the reactor core, and V is the volumeof the reactor core. Further, as can be seen most clearly in the case ofa thermal neutron reactor where most fissions occur in the thermalenergy range, the average neutron flux <ϕ>, the average fission crosssection <σ_(f)> and the thermal fission rate R_(f) will influence thetotal average reactor power <Ptot> in the following way

$\begin{matrix}{\left\langle P_{tot} \right\rangle = {{V\left\langle \phi \right\rangle{\sum\limits_{i}\frac{\left\langle \Sigma_{f,i} \right\rangle}{R_{f,i}}}} = {V\left\langle \phi \right\rangle{\sum\limits_{i}\frac{N_{i}\left\langle \sigma_{f,i} \right\rangle}{R_{f,i}}}}}} & (2)\end{matrix}$where the summation index i runs over the fissile isotope species in thereactor core, and N_(i) is the number of the i^(th) fissile isotopespecies per unit volume. The thermal fission rate R_(f,i) I for thei^(th) fissile isotope species is defined as the rate of fission forthat isotope species to produce 1 W of thermal power, and is inverselyproportional to the thermal energy per fission E_(i) for that species.

Referring to FIG. 1A, a schematic illustration of one example of amonitoring system 100 is shown adjacent a nuclear reactor 102. Thesystem 100 may be used in combination with a variety of suitable nuclearreactors 102, including, for example, the Canada Deuterium Uranium(CANDU) reactor, boiling water reactors (BWR), pressurized waterreactors (PWR), pressurized heavy water reactors (PHWR) and the like. Inthe illustrated embodiments, the nuclear reactor 102 includes a reactorcore 104 that contains nuclear fuel bundles 106 holding fissile nuclearfuel material. When in use, reactions within the reactor core 104produce free neutrons 108 (which may include a combination of fastneutrons and thermal neutrons) and other particles. Some of the neutrons108 are consumed in the fission process, but some neutrons 108 tend toescape the core 104. In the illustrated example, the reactor 102 alsoincludes a radiation shield 110 that surrounds the core 104 and isintended to impeded and/or block the escaping neutrons 108 and reducethe likelihood that neutrons 108 will escape the beyond the radiationshield 110 and into the environment surrounding the reactor 102. Theradiation shield 110 may be formed from any suitable material, includingconcrete, lead and the like and may be of any suitable configuration fora given reactor 102.

While intended to inhibit the escape of neutrons 108, the radiationshield 110 on a given reactor 102 may not be 100% effective, and aquantity of neutrons 108 a may travel beyond the radiation shield 110when the reactor is in use. The inventors have discovered that thecharacteristics of the escaping neutrons 108 a can be measured andcorrelated with the traits/characteristics within the reactor core104—and in particular with the composition of the nuclear fuel bundles106. Optionally, the correlation may also be based on other reactorparameters, including its power output.

The monitoring system 100 described herein can optionally be configuredto detect at least some of the neutrons 108 a that have escaped theradiation shielding using one or more neutron detectors 120 that havebeen positioned outside the radiation shield 110 of the reactor 102, andpreferably beyond a stand-off perimeter 122 defined around the reactor102. The shape and location of the stand-off perimeter 122 may bedetermined by the characteristics of a given reactor 102, itssurrounding buildings/structures and the surrounding environment.

Preferably, the system 100 will include one or more radiation detectorslocated outside the radiation shield 110 and configured to detectneutrons 108 a that have passed through the radiation shield 110. Morepreferably, the system may include two or more radiation detectors thatare each positioned outside the radiation shield 110 and are spacedapart from each other. If multiple radiation detectors are used they maybe arranged to provide a detector array. This may help the system 100detect escaping neutrons 108 a (i.e. record a neutron flux) in two ormore locations around the reactor 102. This arrangement may beadvantageous in instances in which the radiation shield 110 isnonhomogeneous, in which case detectors that are positioned the samedistance from the reactor 102, but are spaced apart from each other, mayreceive different neutron flux. In such embodiments, the system 100 maybe configured to monitor neutron flux, and in particular changes in theneutron flux per unit reactor power, rather than focusing on an absolutemeasurement of the quantity of neutrons 108 a detected at each location.

For example, a system 100 may be positioned around a given reactor 102and operated for a period of time to empirically establish a neutronflux per unit reactor power baseline for the reactor 102 (which mayinclude different absolute levels of neutron flux per unit reactor fluxat different ones of the detectors 120). The length of this calibrationtime period may depend upon the nature of the reactor fuel cycle and thestatistical precision of the detector count rate data, but may last forweeks or months. Then, when the system 100 is used to continue tomonitor the reactor 102 going forward, it may detect changes in theneutron flux per unit reactor power (i.e. deviations from the expectedbaseline), and may alert system users to such changes. Optionally, if adeviation from a baseline flux is detected at only one of the radiationdetectors 120 in the system, the system user may be prompted to checkthe neutron flux per unit reactor power at other radiation detectors 120for the same time period. If a change in neutron flux per unit reactorpower is not registered at the other radiation detectors 120, a systemuser may determine that the change in neutron flux per unit reactorpower is not a result of changes in the operating conditions of reactor102, and optionally may investigate further to determine if the changein flux per unit reactor power is due to a local environmental conditionthat may be affecting first radiation detector. Alternatively, if acorresponding change in the neutron flux per unit reactor is detected atmultiple radiation detectors at the same time (i.e. a deviation from theexpected baseline norm is detected, even if the magnitude of thedeviation is different for different detectors), a system user may beled to determine that the change in neutron flux per unit reactor poweris a result of changes in the reactor 102 and optionally may investigatefurther.

In some embodiments, at least one of the radiation detectors may belocated outside the stand-off perimeter 122, and optionally all of theradiation detectors may be located outside the stand-off perimeter. Thismay help facilitate installation and removal of the radiation detectors,as placement of the radiation detectors may not impact, or requiremodification to the operation of the reactor 102. The radiationdetectors used may be any suitable detector, and in the illustratedexample are neutron detectors.

In the illustrated example, the system 100 includes four neutrondetectors 120 for detecting emissions of neutrons originated from thenuclear fission reactions within the reactor 102 (only three are visiblein FIG. 1A, with all four visible in FIG. 1B). The neutrons detected bythe neutron detectors 120 are operable to detect the flux of escapingneutrons 108 a and to generate corresponding outputs signals that arebased on the neutron flux detected. The outputs signals from each of theneutron detectors 102 can be transmitted to a system controller 124 thatcan monitor the output signals, perform additional signal processing ifdesired, compare the detected values of neutron flux per unit reactorpower to reference values (for example based on historic performance ofa given reactor 102). In some implementations, the transmission of theoutput signals may be provided using a wired connection to thecontroller 124. However in other implementations, the signal may betransmitted to the controller wirelessly using any appropriate wirelesscommunication protocol, or in any other suitable manner.

The controller 120 can be configured to receive one or more additionalinput signals, shown using input 126 in FIG. 1A, that may come from thereactor 102 and/or any other sensors, detectors, and the like that maybe used with the system 100. For example, the controller 120 may beconfigured to monitor the power output of the reactor 102.

The controller 124 may also include a memory and may store informationabout a given reactor 102, such as the particular composition of thenuclear fuel that the reactor 102 is supposed to be burning, e.g. asreported to the IAEA or similar regulator. Using a combination ofinputs, including the baseline neutron flux per unit reactor power forthe reactor, the expected fuel composition, the neutron yield expectedfrom the reactor 102 when burning the declared fuel composition andmeasurements of the reactor power output, the system 100 may be able todetermine if a given reactor 102 is operating within its reportedparameters by comparing the actual neutron flux per unit reactor powerwith the theoretical/expected neutron yield that ought to be presentbased on the reactor power output. For example, an analysis of this typemay identify instances in which the reactor fuel composition differsfrom the fuel composition that was reported to the reactor regulator,and may prompt further investigation.

To calibrate for a given reactor 102, the radiation detectors 120 may beset-up and the system 100 can begin monitoring at a time when the actualfuel composition of the reactor 102 is known (e.g. when the reactor isfirst loaded, etc.) and the output power output can be measured. Basedon the known fuel composition and optionally other reactor parameters,the expected changes to the fuel composition of the reactor 102 overtime can be modelled using conventional modelling software, based onapproaches such as Monte Carlo methods modeling three-dimensionalneutron diffusion (heretofore referred to as “core-following simulationsoftware”). A baseline of neutron flux per unit reactor power for thereactor 102 can be measured, and then deviations from the baseline canbe detected if the measured neutron flux per unit reactor power nolonger matches the expected flux based on conducted calculations. Thismay allow a system operator to confirm if a reactor 102 is operating asexpected, or if its performance has deviated in some manner, which maywarrant investigation. The system 100 can, in these situations, beoperated as a compliance and/or audit tool allowing a system operator tocompare determine if a given reactor 102 is operating in accordance withits claimed safety and production margins without requiring the systemoperator to have direct access to the reactor core, spent fuel orcomponents. This may be useful in circumstances when the reactoroperator is unwilling to provide such access, the reactor is dangerousand/or challenging to access physically and/or it is desired to “doublecheck” the reactor operator's reported conditions with an independentmeasurement/determination of the reactor conditions.

The expected neutron flux per unit reactor power for a given reactor 102can be determined (based on reactor core-following simulation data,monitoring the neutron flux per unit reactor power during a calibrationperiod to empirically develop a base line for the reactor, a combinationof these and the like) and stored in the controller. The measuredneutron flux per unit reactor power can then be compared to the expectedneutron flux per unit reactor power, and the controller may generate anoutput based on the comparison. For example, the controller may alert asystem user if the measured neutron flux per unit reactor power differsfrom the expected neutron flux per unit reactor power. The controllermay, optionally, be configured to produce more than one alert level—forexample based on the magnitude of the difference between the measuredneutron flux per unit reactor power and the expected neutron flux perunit reactor power. For example, the controller may generate a lowpriority alert if the measured neutron flux per unit reactor power iswithin about 1% of the expected flux per unit reactor power, and a highpriority alert if the measured neutron flux per unit reactor power ismore than 3% different than the expected neutron flux per unit reactorpower.

While a single detector 120 could be sufficient to provide the desiredmonitoring in some instances (as it can collect the relevant data), in apreferred embodiment of the system 100 multiple detectors 120 would beused.

The neutron detectors 120 used in the system 100 may be any suitabletype of detector that provides a detector signal-to-background ratio onthe order of 10 or more with reactor at operating power. Achieving thiswill depend upon the sensitivity of the detector, the size of thedetector, the incident neutron flux available from the reactor, and thesize of the neutron background in the local environment. Experience withlarge area detectors that are about 10% efficient with active detectionarea on the order of 0.1 m² is sufficient when the incident neutron fluxis on the order of 10⁻¹ cm⁻² s⁻¹, against a cosmic ray-induced neutronbackground on the order of 10⁻² cm⁻² s⁻¹. The use of large-area neutrondetectors may help the neutron detectors 120 capture a desirable portionof the neutron flux, and may help the detectors generate a useful outputsignal even if the quantity of neutrons 108 a escaping the shielding 110is relatively low. For example, in the same neutron flux conditions, asystem utilizing smaller neutron detectors may produce output signalsthat are relatively weaker and/or may have a less desirable signal tonoise ratio.

Some neutron detectors may be more effective at detecting thermalneutrons than fast neutrons (other operating conditions being equal). Asthe radiation shielding 110 surrounding the reactor core 104 is alsobelieved to be more effective at stopping thermal neutrons than stoppingfast neutrons, it is anticipated that the neutrons 108 a escaping theshielding 110 may have a relatively higher proportion of fast neutronsto thermal neutrons as compared to the neutron flux within the core 104.Optionally, the system 100 may include a moderator positioned betweenthe radiation detector 120 and the reactor 102 to help slow neutrons inthe neutron flux 108 a, and to convert at least some of the fastneutrons into thermal neutrons that can be more easily detected. Themoderator may be any suitable moderating material, including a liquid ora solid, and optionally may be formed from high density polyethylene(HDPE). One example of a moderator 128 is schematically illustrated inFIG. 1B as an object positioned one of the detectors 120 and the reactor102, but in other examples the moderator 128 may be provided as part ofthe radiation detector 120 instead of being provided as a separatemember. The moderator 128 may have any suitable configuration, and mayhave a moderator thickness 130 (i.e. the thickness of the moderatormaterial in the direction the neutron flux travels through the moderator128 to reach the detector) of between about 0.1 inches and about 5inches or more, and optionally may be between about 0.5 inches and about3 inches and may be about 1 inch. A plot showing some measured detectorcount rates vs moderator material thickness (for a high densitypolyethylene moderator) illustrates a peak detector count rate at about1 inch (about 2.5 cm) of moderator, where zero thickness corresponds tothe absence of the moderator 128. This result helps confirm the beliefthat the neutrons 108 a that are leaking beyond the shielding 110 mayhave a relatively high proportion of fast neutrons, and that using asuitable moderator layer may help improve measurement efficiency (i.e.help increase detector counts), whereas providing a moderator layer thatis relatively thinner or relatively thicker may reduce the detectionefficiency. This may also be understood from the point of view that theneutron energy spectrum in a thermal (moderated) reactor core contains athermal spectral component (peaked near thermal equilibrium energy) anda fission spectral component (with average energy near 2 MeV in somecircumstances) which may be bridged by an epithermal regime. Absorptionof neutrons within the reactor core may preferentially select low energyneutrons, possibly due to their higher absorption cross section relativeto higher energy neutrons. The higher energy neutrons within the reactorcorrespondingly may have a higher leakage probability. FIG. 7 isconsistent with the detector on average receiving higher energy neutronsthat have been partially moderated by the detector's exteriorenvironment.

Alternatively, instead of using a moderator 128 to convert fast neutronsto thermal neutrons, the system 100 may include radiation detectors thatare configured to directly detect fast neutrons, and/or a combination offast neutron detectors and thermal neutron detectors.

One example of a suitable large-area neutron detector is a Boron CoatedStraw (BCS) detector made by Proportional Technologies Inc. (Houston,Tex., USA). FIG. 2 shows an illustrative, schematic diagram of thestructure of a one example of a BCS detector 200. Such detectorsgenerally comprise a housing 202 with a number of sealed aluminum tubes204 arranged in parallel. Within each aluminum tube 204, are copper“straw” tubes 206 arranged in a bundle. As shown in FIG. 3 , each strawtube 206 coated with ¹⁰B-enriched boron carbide (¹⁰B₄C) and a tensionedwire 208 along the long axis of each straw tube. In an exampleconfiguration the BCS detector may have seven aluminum tubes may bearranged in parallel and within each aluminum tube, seven additionalcopper “straw” tubes are arranged within providing a total of 49 strawtubes. Each straw may be sealed with an end cap and the interior of thestraw tubes be filled with gas such as Ar/CO₂ gas (90/10 ratio) at 10.5psi.

Another example of a large-area neutron detector consists of sevensealed “B10+” stainless steel tubes (2.45 cm diameter, 101.6 cm activelength) from General Electric Reuter Stokes (Twinsburg, Ohio, USA) linedwith an elemental ¹⁰B-enriched coating, and filled with 0.75 atm ³He,along with Ar and CO₂, to a total pressure of 16.22 psi. The tubescollectively present an active area of 1 m×0.18 m on the broadest sides.

Referring still to FIG. 3 , thermal neutrons captured in the ¹⁰B coatingmay be converted into secondary particles, through the ¹⁰B(n,α) reactionas described in the formula below (the probability of a given reaction'soccurrence denoted within the brackets):n+ ¹⁰B→⁷Li*+α+2.3 MeV (94%)  (3)n+ ¹⁰B→⁷Li+α+2.8 MeV (6%)  (4)

The secondary ⁷Li and α (or ⁴He nucleus) particles may be emitted inopposite directions, isotropically, as dictated by the conservation ofenergy and momentum. One of the two charged particles may enter thestraw tube and ionize the gas contained within the straw. For each strawtube, a voltage can be applied such that the tube wall can act as acathode while a thin wire tensioned within its center can be operated asan anode electrode. As a result of gas ionization, electrons liberatedin the gas can migrate from the cathode to the anode. During thismigration avalanche multiplication of electrons can result as a resultof an electric field produced by the voltage across the straw tube sothat a detectable electrical charge can be measured using appropriateelectronics.

Thermal neutrons may also be captured in ³He gas through the ³He(n,p)³Hreaction as described in the formula below:n+ ³He→³H+p+0.764 MeV  (5)

For each proportional counter tube, a voltage can be applied such thatthe tube wall can act as a cathode while a thin wire tensioned withinits center can be operated as an anode electrode. With ³He gas fillingspace between the anode and cathode, its reaction products instigate gasionization wherein electrons liberated in the gas can migrate from thecathode to the anode. An electrical charge can be measured usingappropriate electronics, as a result of this avalanche multiplication ofelectrons.

For example, a charge sensitive preamplifier and shaping circuitry canbe used to produce a low-noise pulse for each neutron event. Theelectronic pulses can be captured, for example, using a data acquisitioncard. Each pulse detected may correspond to a “detector count”. Thus,the number of detector counts over a period of time may be used todetermine a detector count rate. The detector count rate can be comparedagainst a corresponding measured detector thermal reactor power.Accordingly, having a priori knowledge of the isotopic-dependentproperties of various reactor fuel materials, detection of changes inisotopic composition of a reactor core may be possible based on thedetector counts and detector count rates.

To test the performance of one embodiment of a system 100, one exampleof a system 100 was deployed to detect of neutron escaping during theoperation of the National Research Universal (NRU) reactor, located atthe Chalk River Laboratories (Chalk River, Ontario, Canada) operated byCanadian Nuclear Laboratories Inc. The NRU reactor is heavy water cooledand moderated, with online re-fueling capability. It is licensed tooperate at a maximum power of 135 MW, and has a peak thermal flux of4.0×10¹⁴ n/cm²/s.

The NRU reactor consists of various types of rods, including driver fuelrods, Mo-99 and Co-60 production rods, absorber rods, and control rods.The NRU driver fuel is a low-enriched uranium (LEU) fuel alloy of Al-61wt % U₃Si, in which U₃Si particles are dispersed in a continuousaluminum matrix, with 19.8% U-235 in uranium.

BCS detectors, as described previously comprising a total of 49 strawtubes, were deployed for neutron detection. Each straw tube may bebiased at a high voltage to establish the necessary electric field togenerate the electron avalanche. For example, in the experimental setup,the straw tubes were be biased with +1000V using a high voltage supply.The 49 straw tubes can each provide a signal output such that and eachsignal output can be added together using a summing amplifier. A DCpower supply can be used to provide +/−5 V to each of the signal outputends, and the summing amplifier.

The output of the summing amplifier can be further processed usingappropriate signal conditioning components. For example the summingamplifier output can be first processed using a shaping amplifier (Ortec671 shaping amplifier), a channel analyzer (an Ortec 406A single channelanalyzer), and an Ortec 416A gate and delay generator. The processedsignal can be provided to a data acquisition system such as a NationalInstruments (NI) cRIO-9023 real-time controller through a NI 9402 LVTTLhigh-speed bidirectional digital I/O module. The NI cRIO-9023 can beconfigured to provide time-stamping of individual pulses that correspondto detected neutron events. Time-series plots of captured pulses may beused to provide a record of detector counts as a function of time, whichpermits examination of changes in count rate (i.e. neutron detectionrates) during measurement.

The B10+ detector tubes are biased to +700 V. The high voltage issupplied by a NPM3100E neutron pulse monitor (NPM) from QuaestaInstruments (Tucson, Ariz., USA), which also processes pulses through acharge sensitive amplifier, a fixed gain pulse-shaping amplifier, avariable gain amplifier, and an analog to digital converter, beforeusing firmware algorithms to analyze the digitized data. The NPM wasused to record time-stamped pulses in list mode.

For the experimental setup, the system 100 included a detector arrayhaving two neutron detectors at two different locations in proximity toNRU, outside the reactor shielding and at a stand-off distance. A firstplacement location (referred to as location A) corresponded to aposition within the NRU reactor building, located approximately 17 mfrom the NRU reactor core and two levels below the main reactor floor. Asecond placement location (referred to as location B) corresponded to aposition outside of the NRU reactor building in another structure (aportable trailer building), approximately 69 m from the NRU reactorcore. Each of the BCS neutron detectors in the experiment were coupledto the data acquisition system to produce a count rate as a function oftime during the course of measurement and then compared against NRU'smeasured thermal reactor power. The detectors were operated over twotime periods, from November to December 2014 and from April to August2016.

For each neutron detector, its count rate as a function of time wasrecorded during the course of the measurement. This data is comparedagainst NRU's simultaneously measured thermal reactor power, asillustrated in FIGS. 12 and 13 . The NRU reactor typically undergoes ascheduled shut-down and subsequent start-up every few weeks ofoperation, which also occurred during the course of neutronmeasurements. The shutdown and startup procedures provided anopportunity to assess the sensitivity of the BCS and B10+ neutrondetectors to reactor shutdown and startup. FIGS. 4A and 4B show that thecount rate of the B10+ detector at locations A and B, respectively as afunction of average reactor power (MW) during reactor shut-down andsubsequent start-up. FIG. 12 shows that the neutron detector count rategenerally correlates with the thermal reactor power, including duringthe course of reactor start-up and shut-down. Some background count ratecan be seen when the reactor is shutdown, due to neutrons from cosmicray background radiation. FIG. 12 also shows that the neutron detectioncount rate generally follows the average reactor power through itstemporal fluctuations while the NRU is at power. This suggested to theinventors that the ratio of reactor power to detector count rate is ameaningful quantity to follow. FIG. 13 demonstrates how the ratio ofreactor power to detector count rate remains relatively constant whilethe reactor is at power

FIGS. 4A and 4B are plots of measured neutron flux detection rate vs.reactor power, and shows a correlation between detector count rate andNRU's thermal reactor power during the course of reactor startup andshutdown. It was noted that the signal (e.g. from the NRU reactoroperating) to background (e.g. when NRU reactor is shutdown) ratioranges from about 7:1 to 10:1 at location B.

During the testing it was noted that the signal from location A is lessthan half in terms of the number of counts as compared to location B,even though location A was positioned at a closer to the reactor. It isbelieved that this may be attributable to increased shielding andoverburdening for neutrons to reach location A, as compared to reachinglocation B.

The experimental measurements may also be compared with simulatedneutron emission based for the same measurement periods noted previously(November to December 2014 and from April to August 2016). Massinventory of the fissile plutonium (Pu) and uranium (U) isotope of thereactor core can be extracted from the simulation for the periods. Theratio of neutron flux per unit of reactor power can be calculated fromsimulated fissile isotope mass inventors using the followingrelationship:

$\begin{matrix}{\frac{\phi}{P_{tot}} = \left\lbrack {N_{A}{\sum\limits_{i}\frac{m_{i}\sigma_{f,i}}{w_{i}R_{f,i}}}} \right\rbrack^{- 1}} & (6)\end{matrix}$where, ϕ=Average neutron flux; P_(tot)=Total reactor power;N_(A)=Avogadro's Number; m_(i)=Mass of i^(th) fissioning species;σ_(f,i)=Average fission cross section of i^(th) fissioning species;w_(i)=Atomic weight of i^(th) fissioning species; R_(f,i)=Fission rateof i^(th) fissioning species for 1 W of reactor power.

It has been discovered by the inventors that the neutron detection countrate has a substantially linear dependence on average nuclear reactorpower. This linear dependence is exhibited in FIG. 4B, which shows alinear regression fit applied to the B10+ detector count rate atLocation B as a function of average reactor power, during reactorstart-up and shut-down periods. It is noted that the neutron detectorcount rate shown is not corrected for any environmental influences,whether from changing atmospheric conditions or changing operationalenvironment; the contribution of these factors is in the minimal scatterpresent in the figure.

FIG. 5 shows the experimental data (detector count/reactor power) versussimulated data (neutron flux/reactor power). The simulated data for thiscomparison was generated using core-following neutron diffusionsimulation software called TRIAD (T. C. Leung and M. D. Atfield,“Validation of the TRIAD code used for the neutronic simulation of theNRU reactor”, Proc. 30^(th) Annual Conference of the Canadian NuclearSociety, Calgary, Alberta, Canada, May 31-Jun. 3, 2009). The error barsare standard deviations of the mean, each taken over the period of aboutone month. In this plot, the experimental data is displayed relative onthe y-axis, against the simulation data displayed relative on the rightx-axis. A linear fit has been applied to the experimental data shown,demonstrating a linear relationship between the detector count rate perunit reactor power and the calculated neutron flux per unit reactorpower.

In view of the foregoing, changes in the detection rate, whichcorrelates with changes in reactor core power can be used to followvariations in reactor power. For example, isotopic-specificcharacteristics of various isotopes as shown in Table 1 above, can beused to infer changes in reactor composition based on time-basedvariations in detection rate. Specifically, for the describedexperimental detection array, variations in reactor power (up to ˜100MW) can be measured with a signal to noise ratio of up to 10 to 1.Furthermore, variations in isotopic composition in the reactor core canalso yield about a 10% change in neutron flux per unit reactor powerthat is discernable from time-dependent neutron detection count ratedata. As such, analysis of the variation in detection rate may providefurther information about the reactor, in particular, an indication ofthe changes in isotopic composition within the reactor. This can beachieved, for example, by examining the direction and extent of how themeasured detector count rate per unit reactor power deviates from theexpected value. The expected value is based on baseline data fromprevious calibration measurements. This deviation can be corroboratedagainst and interpreted by knowledge of the initial reactor compositionfrom a reference date prior to the deviation occurring, and utilizingreactor core-following simulation data. These deviations are directsignals of changes in fission rate. The interpretation of these changesin terms of changes in mass inventory of the reactor core is dependentupon baseline data both from empirical neutron detector measurement, andfrom calculations with core-following Monte Carlo simulation software.

Another application of the system and methods described herein may be asa tool for independent verification purposes in safeguards applications,as the system may be sensitive to changes in fissile isotope inventoryin the reactor core. As an example a simplified model was employed torelate the neutron flux per unit reactor power to reactor parametersthat depend upon the fissile isotope inventory of the reactor core. Thismodel assumed that the neutron flux go in the core is an appropriatespace and energy averaged value, and that the fission cross section forthe i^(th) fissioning species is a corresponding average cross sectionIn a real reactor, the neutrons are not monoenergetic, and for eachneutron energy there is a corresponding flux and cross section that canvary with time and position in the reactor. Instead, the simplifiedmodel here uses the aforementioned average quantities on the groundsthat most of the fissions in a thermal reactor occur in the thermalenergy region, where both the neutron flux and fission cross sectionvalues are large.

In the simplified model above, the quantities that vary in equation (1)over this time scale are the fissile isotope masses m_(i), particularlyas the ²³⁵U isotopes are burned up and produce other isotopes of U andPu. As the NRU reactor undergoes online re-fueling on a routine basis,the fissile isotope inventory of the reactor core normally remainsrelatively constant. However, in the recent past, the extent of thefueled region in the reactor core was increased, and the quantity of⁵⁹Co absorber in the core was significantly increased for production ofhigh specific activity ⁶⁰Co. As a result of these changes, the NRUreactor required more frequent re-fueling, and consequently a smallerfraction of the uranium present in the fuel is consumed. Consequently,the maintained uranium mass inventory relative to the plutonium presentin the core increased as a result of these changes. Using the massinventory numbers provided by TRIAD over time, up to a 10% reduction inneutron flux per unit reactor power is predicted by Equation (6). Thisis shown in FIG. 5 , along with a linear regression fit with a slope of0.60±0.11 cpm cm² s, and a y-intercept of 0.082±0.13 cpm/MW. The errorbars shown are standard deviations of the mean, each taken over theperiod of a month.

Table 2 below shows the changes in average fissile isotope masses in theApril to August 2016 time frame, relative to the 2014 November toDecember time frame, corresponding to data shown in FIG. 5 .

TABLE 2 MASS Mass Percent Change DIFFERENCE Relative to Total IsotopeISOTOPE (kg) Mass (%) 235_(U) 2.864 11.70 238_(U) 75.114 34.08 239_(Pu)0.07157 15.21 241_(Pu) −0.00993 −36.45

On another occasion, the start-up of pressurized test loop facilitiesconnected to the core of NRU was coincident with substantial increasesto the Pu content of the core, while the content of U isotopes in thecore each did not change by more than 3%, as described in Table 3(a)below, which shows the change in average fissile isotope masses in theSeptember to October 2016 time frame, relative to the 2016 October toNovember time frame. As summarized in Table 3(b), which shows a changein measured neutron count rate at Location B per unit reactor power, andpredicted in-core neutron flux, corresponding to isotopic changes shownin Table 3(a), these changes in isotope content resulted in significantchanges in neutron count rate per unit reactor power (averaged over afew weeks) that are corroborated with significant changes in predictedneutron flux per unit reactor power, based on TRIAD data.

TABLE 3(a) MASS Mass Percent Change DIFFERENCE Relative to Total IsotopeISOTOPE (kg) Mass (%) 235_(U) 0.529 2.10 238_(U) −13.1 −2.80 239_(Pu)0.173 15.1 241_(Pu) 0.166 41.6

TABLE 3(b) B10+ Detector Estimated count rate per unit in-core neutronreactor power, Location flux Quantity B (cpm/MW) (cm⁻² s⁻¹ MW⁻¹) Beforestart-up 6.646 ± 0.088 (1.1445 ± 0.0027)E+07 of test loop After start-up6.263 ± 0.073 (1.1098 ± 0.0024)E+07 of test loop

As shown in FIG. 5 and Tables 2 and 3(a) and 3(b) the inventors notedthat a relatively small reduction, such as a few percent, in the neutronflux per unit power of the NRU, due to changes in the isotopic inventoryin the reactor core, contributed to a relatively significant detectorcount rate per unit reactor power while at a stand-off distance from thereactor core (seen at both locations A and B). This helps demonstratethat the technique of stand-off reactor monitoring using neutrondetection may be used as means of independently verifying when there isa change in the fissile isotopic inventory of a thermal nuclear reactor,in some operating conditions. For example, the data shown in Tables 2and 3(a) and 3(b) above demonstrate that with neutron detection atstand-off distances outside of a reactor core, it is possible to measurethe movement of kilogram, and even sub-kilogram quantities of fissile Puand U isotopes.

It is also noted that the above performance may be affected by thedetection efficiency of the particular neutron detector that isemployed, and/or the environment in which the detector is placed. Theenvironmental factors may include the size of the reactor and theneutron flux that it produces, the nature and extent of the reactorshielding and the neutron flux leakage that it permits, theoverburden/influence of other building infrastructure that may existbetween the exterior of the reactor shielding and the neutron detector'slocation, and the changing operational environment.

For example, to examine the influence of the local environment, FIG. 3compares BCS detector data acquired at Location A with B10+ detectordata taken simultaneously at Location B. From FIG. 14 , it is evidentthat the B10+ detector data at Location B is about 7.5 times greater onaverage than the BCS detector data at Location A. Although the B10+detector may be more efficient at detecting neutrons than the BCSdetector by nearly a factor of 2, Location A also presents substantiallymore environmental shielding and overburden than Location B in thecurrent experimental set-up. The shielding infrastructure between thereactor core and Location A presents roughly 7 m of high densityconcrete flooring and shielding, 0.5 m of steel side thermal shield, and0.5 m of water reflector. Many neutrons located at Location B may haveleaked through the top deck plate of reactor and may scatter in the mainreactor hall before escaping through the exterior walls and windows ofthe reactor building; leaking through the top of the reactor presentedapproximately 1.2 m of steel and 3 m of water in shielding.

FIG. 14 also shows that Location B data exhibits regular 50% decreasesin detector signal, while the Location A data does not. FIG. 15 showsthat in spite of these regular 50% deviations in signal at Location B,the simultaneous signals from Locations A and B correlate very well witheach other. In fact, the 50% deviations in detection rate at Location Bmatch the timing of online refueling activities occurring at the top ofthe reactor, as detailed in FIG. 16 . Although the reactor power maydecrease during online refueling, it does not always do so, and the B10+count rate drops well before there is any change in reactor power, ifthere is any. Rather, the drop in B10+ count rate may result from thefuel rod flask blocking neutrons escaping from the top of the reactor,when the flask is positioned over top of the reactor during reactor rodmovement. The spikes in count rate often observed in the middle of thedrop in B10+ count rate coincide with freshly irradiated fuel rodsmoving out of the reactor core into the flask, and decaying over a 30min period. These events were not recorded at Location A, as theposition of this location and the significant amount of fixed shieldingpresented to this location prevents a detector at Location A from seeingneutrons from the top of the reactor.

Because of these local environmental effects on the performance of thestand-off monitoring system, such as the fact that detector at LocationB appears to have been able measure online refueling events, while adetector at Location A could not, it may be useful in some embodimentsof the system to ensure that multiple detectors (i.e. two or more) areplaced at multiple locations around a target reactor or other object tobe measured. This may allow the detection signals from each detector tobe compared with each in order to help discriminate meaningful changesin the reactor's output from local events, background emissions andother interference.

Referring now to FIG. 6 , one example of a method 600 of using a system,such as system 100, to determine the isotopic composition of the fuel ina nuclear reactor core, when isotropic properties of materials withinreactor is known (e.g. the properties listed in Table 1). At step 602,at least one, and preferably two or more, neutron detectors 120 such asthe BCS detectors may be distributed around the target reactor andoutside its radiation shield, optionally at a standoff distance. Forexample, as shown in FIG. 1 the detectors 120 may be distributed aroundthe reactor core 104. In some other configurations, some detectors maybe provided below the director core, at a standoff distance, within thereactor building.

With the detectors 120 in place, the system may, at step 604, determinean expected neutron flux per unit reactor power for the reactor beingmonitored, and may store this in a controller memory or the like. Thisexpected neutron flux per unit reactor power can serve as the valueagainst which the measured neutron flux per unit reactor power can becompared. This expected neutron flux per unit reactor power may beobtained from any suitable source, including from computermodeling/simulation data and/or an empirically derived neutron fluxbaseline as described herein, and the like. In the example method 600,the expected neutron flux per unit reactor power is illustrated as beingobtained from either step 606, monitoring the neutron flux per unitreactor power received at the stand-off location over a calibrationperiod to help determine a neutron flux per unit reactor power baselinefor the target reactor, or step 608, based on data received from acomputer simulation (e.g. TRIAD) of the operation of the target reactorin accordance with its reported operating conditions and determine aneutron flux per unit reactor power baseline for the target reactor fromthe simulation data. While two options are shown, other methods ofobtaining the expected values of neutron flux per unit reactor power maybe used.

For step 606, the detectors 120 may optionally be calibrated bydetecting neutron emissions from the detector core 104 for a definedcalibration period. In some cases, as discussed above, the calibrationmeasurement period may be during a restart of the reactor 102 or aftermaintenance (or replacement of reactor fuel 106) and may last until thereactor 102 is shut down again for maintenance. In other cases, themeasurement period may be a period of time between restart andsubsequent shut down procedures. In yet other cases, the measurementperiod can last for several restart and subsequent shutdown periods.Measurements during this time period may help establish a baselineneutron flux per unit reactor power for a given reactor 102.

At step 610, the system 100 can be used to monitor the neutron flux(i.e. the detector counts) received at the stand-off location and thereactor power output over an active monitoring time period (i.e. a timeperiod in which the compliance inspection of the reactor is desired).The data from the detectors 120 during the active monitoring time periodcan be logged and processed so that a detector count rate (e.g. numberof detector counts per unit of time) can be determined for each detector120.

At step 612, having determined the detector count rates, the system 100can compare the measured neutron flux rates per unit reactor poweragainst the expected neutron flux rates per unit reactor power. Ifdifferences of a sufficient magnitude are noted, the system may generatean alert (step 614), indicating that the target reactor appears to beoperating in a manner that deviates from its expected operatingconditions. This may be considered a relative type of analysis, in whichthe absolute value of the neutron flux per unit reactor power need notbe directly reported to a system user, and instead the alert can bebased on the relative deviation of the measured neutron flux per unitreactor power from the expected neutron flux per unit reactor power.Alerts of this nature may be at least one criteria that a safeguardsinspector may use to determine (for example, through remote monitoring)if further investigation of the target reactor and its operatingconditions are required.

Optionally, instead of, or in addition to, the relative, comparativetype analysis, the system may also be configured to analyze the measuredneutron flux, and associated reactor power output, and compare itagainst known fission rates of various fuel materials and compositions(such as the baseline measurements, reactor core-following simulationdata and the like). For example, changes in detector count rates may beused to indicate a transformation of one type of fissile material toanother within the reactor core 104. In such configurations, the system100 may be able to determine the isotopic composition of the fissilematerial within the target reactor, and this information may then becompared to the records or simulations that set out the expected nuclearfuel composition (based on the reported reactor operating parameters).Optionally, some of the method steps, such as steps 604-612 and/or610-612 can be performed iteratively (at any desired sampling rate), asindicated using dashed arrows.

Optionally, the method 600 may be performed independently for eachdetector 120 in the system. Alternatively, the outputs from multipledetectors 120 may be considered together by the controller 124. Forexample, steps 610-612 may be performed for each detector 120 and theirresults noted by the controller 124. If each detector 120 has recorded aneutron flux within the expected range, the controller 124 can determinethat no alert condition is present. If, alternatively, if one detector120 records an anomalous neutron flux reading, the controller 124 mayquery other detectors 120 (as explained herein) to determine if theyalso detected a deviation from the expected neutron flux (even if of adifferent magnitude than the first detector) before triggering an alert.Optionally, the controller 124 may also re-query the detector 120 thatrecorded the anomalous reading at a later time to determine if theunexpected reading is persistent or was merely transient. Both of thesetechniques, amongst others, may allow the controller 120 to determine ifa deviation in the neutron flux is a result of a change in reactorconditions (an event that should trigger an alert) or is a result ofenvironmental effects or other non-reactor related events (an event thatought not to trigger an alert).

Stand-off neutron monitoring using, optionally, large area neutrondetectors may be used to monitor sites or containers whereneutron-emitting ²³⁸U or Pu is stored. Such sites could in principleinclude used fuel dry storage containers or sites, fuel recycling andreprocessing facilities, and mixed oxide fuel fabrication facilities.

To help evaluate examples of such monitoring systems, this disclosurealso describes a Monte Carlo model of a large-area neutron detector atvarious locations around the ZED-2 research reactor at Chalk River, ON,Canada. MCNP6 has been employed to construct and evaluate the MonteCarlo models of a large-area neutron detector at several locationsoutside of the reactor shielding of ZED-2 as discussed herein. Thelarge-area detector in the model is based on a boron-lined neutrondetector from Proportional Technologies, Inc. (Houston, Tex., USA),which has an active cross-sectional area of 1 m×0.18 m. The modelleddetector, which can be referred to as a Boron-Coated Straw (BCS)detector, consists of seven sealed aluminum tubes (2.54 cm diameter, 1 mlong), each of which consists of 7 10B-enriched B4C coated strawdetectors (7.5 mm in diameter, 1 m long). Each straw is filled withAr/CO2 gas (90/10) at 10.5 psi. The detector has a total of 49 sealedstraws. This detector has been used to acquire neutron data signals atlocations around the ZED-2 reactor; the experimental data is used tocompare against simulation results reported here.

The simulation approach is divided into three smaller simulations(stages) that each build upon the previous simulation's calculatedneutron field. Specifically, neutron tracks that pass through designatedsurfaces of one simulation are recorded and are used as the sourceneutrons of the next simulation. In MCNP6, the cards to do so are thesurface source write card (SSW) and the surface source read card (SSR).

Stage 1 is the core of the reactor, which is where the core neutron fluxis calculated. The neutron flux from fission in the reactor core isgenerated from a kcode criticality calculation. Stage 1 is kept as aseparate stage to facilitate changing of core models. Stage 2 is thebuilding model—consisting of the reactor shielding, two rooms in thereactor building and volumes to contain detectors. Stage 3 is used totransport neutrons from the volumes surrounding the BCS detector intothe detectors themselves and perform flux tallies of heavy ions producedfrom neutron capture in the boron coating of the BCS detector. There arecurrently three locations for detectors in the models. Stage 3 wasdesigned as a separate stage so that changing detectors in subsequentmodels/experiments may be relatively easier (i.e. using a differentdetector in place of the BCS or rotating the detector orientation) andso that heavy ion and alpha particle physics can be turned off for thefirst two stages of the model.

The stage 1 core model is based on the model documented inZED2-HWR-EXP-001 in the International Reactor Physics BenchmarkEvaluation Project, and was modified to include all of the fuels andcomponents present in the core during neutron detector data acquisition.Stage 1 may help facilitate the generation of a fission distribution inthe critical reactor core, and determine a resulting neutron fluxdistribution. The flux distribution is saved as a list of neutron tracksfor use as a source in subsequent stages of the calculation Stage 1simulates the ZED-2 core, moderated with heavy water. The core ispartially surrounded by a graphite reflector, all of which is containedin a concrete enclosure. The concrete shielding has been simplified inthe model by, for example, not including vents or the holes where pipesgo through the shielding.

Stage 2 is a building model 200. It contains the concrete shielding 202around the reactor 204, the ZED-2 facility, and an adjacent room whereneutron measurements were conducted. Technical drawings of the reactorand the surrounding building were used to get the dimensions andmaterial compositions for this stage of the model. The neutrons recordedin stage 1 are read onto a surface inside the reactor shielding 202. Thevolume inside of the cylinder that they are read to, used to define thereactor 204, is voided (given a neutron importance of 0) so that theneutrons are not double counted. The purpose of this stage is totransport the neutrons from the core to the three locations wheredetectors have been set up. A challenge in this stage involves gettingenough neutrons through the concrete shielding to have acceptablestatistics inside the detectors. Variance reduction using weight windowsgenerated by the ADVANTG software package (Oakridge National Laboratory)was employed to alleviate this problem. The results of stage 2 arewritten to three separate hemispherical surfaces filled with air, eachat positions where the BCS neutron detector has experimentally collecteddata.

The particles written to the three surfaces in stage 2 form the sourcefor stage 3, which places a model BCS detector 206 in the enclosedvolume of each surface. An MCNPX model of the BCS detector has beenadapted into MCNP6, and employed in the said surface volumes. Fluxtallies (F4 tallies) over the detector cells in stage 2 and currenttallies (F1 tallies) in stage 3 can be compared to the experimental datato gauge how well the simulations match reality.

Simulations were run using low-enriched uranium (LEU) and naturaluranium (NU) ZED-2 cores implemented in stage 1 of each simulation. Inthe kcode criticality calculation of stage 1, 6×10⁶ neutrons weregenerated per cycle for 120 cycles, neglecting the first 20 cycles.Model neutron detectors 206 were positioned in locations immediatelyeast and south of the reactor walls of ZED, and near a wall in room 208adjacent to the south wall of the ZED-2 reactor room, as shown in FIG. 8.

The simulated neutron energy spectra recorded at the model neutrondetectors are shown in FIGS. 9 and 10 . Some trends evident in thesespectra are brought out in FIG. 11 , which displays area sums underthree spectral regions: thermal (<3×10⁻⁷ MeV), epithermal (3×10⁻⁷ to<3×10⁻² MeV), and fast (3×10⁻² MeV). It can be seen that epithermal andfast region sums are larger for the East Side of the reactor than forthe South Side, and larger for the LEU core than for the NU core. Thethermal region area sums appear to show the inverse trends to the abovetrends, but the size of the error bars for the thermal region area sumspreclude any definite conclusions.

TABLE 4 Measured Calculated Flux Flux Diff. Core Location (cm⁻² s⁻¹ W⁻¹)(cm⁻² s⁻¹ W⁻¹) (%) NU Adjacent 0.25 ± 0.03 0.213 ± 0.003 −14 Room LEUAdjacent 0.363 ± 0.04  0.443 ± 0.003 22 Room LEU East Side 1.1 ± 0.12.74 ± 0.07 139 LEU South Side 2.469 ± 0.3  3.3 ± 0.3 35

Table 4 compares reactor-power normalized neutron flux at the detectorlocations from simulation to that from experiment. The experimental datawas flux normalized by reactor power, calculated from neutron countrates measured by a BCS detector. The simulated reactor power wascalculated in order to normalize the simulated flux per unit power. Tothis end, a fission energy deposition (F7) tally was tabulated over allfuel in stage 1. The tally result was changed from MeV to Joules, andthen multiplied by the ratio of total usable energy per ²³⁵U fission tothe amount of prompt energy MCNP6 counts per ²³⁵U fission. This correctsthe F7 tally to give the total energy the reactor releases instead ofjust prompt fission energy.

Two examples of neutron detectors that may be used in the systems andtesting described herein are explained below. One example of alarge-area neutron detector is a boron-lined detector from ProportionalTechnologies, Inc. (Houston, Tex., USA), which has an active area of 1m×0.18 m on its broadest sides. The detector, heretofore referred to asa Boron-Coated Straw (BCS) detector, consists of seven sealed aluminumtubes (2.54 cm diameter, 1 m long), each of which consists of 7¹⁰B-enriched B₄C coated straw detectors (7.5 mm in diameter, 1 m long).Each straw is filled with Ar/CO₂ gas (90/10) at 10.5 psi. The detectorhas a total of 49 sealed straws. The straws are biased with a +1000 Vhigh voltage supply. The 49 straws provide signal output at each end ofthe detector tubes, and these signal outputs are added together using asumming amplifier. A DC power supply provides +/−5 V to each of thesignal output ends, and the summing amplifier. The output of the summingamplifier is relayed via an Ortec 671 shaping amplifier, an Ortec 406Asingle channel analyzer, and an Ortec 416A gate and delay generator to aNational Instruments (NI) cRIO-9023 real-time controller through a NI9402 LVTTL high-speed bidirectional digital I/O module. The NI cRIO-9023provides time-stamping of individual pulses.

Another example of a large-area neutron detector consists of sevensealed “B10+” stainless steel tubes (2.45 cm diameter, 101.6 cm activelength) from General Electric Reuter Stokes (Twinsburg, Ohio, USA) linedwith an elemental ¹⁰B-enriched coating, and filled with 0.75 atm ³He,along with Ar and CO₂, to a total pressure of 16.22 psi. The tubescollectively present an active area of 1 m×0.18 m on the broadest sides,and are biased with +700 V. The high voltage is supplied by a NPM3100Eneutron pulse monitor (NPM) from Quaesta Instruments (Tucson, Ariz.,USA), which also processes pulses through a charge sensitive amplifier,a fixed gain pulse-shaping amplifier, a variable gain amplifier, and ananalog to digital converter, before using firmware algorithms to analyzethe digitized data. The NPM was used to record time-stamped pulses inlist mode.

To help facilitate data acquisition when using either type of detector,acquired binary list mode files may be off-line binned into one or moretime-series histogram via C++ routine. The time-series plots may providea record of detector counts versus time, which may help facilitate theexamination of changes in count rate that occur during measurement. Thedetector count rate as a function of time during the course ofmeasurement may be compared against NRU's measured thermal reactor poweras a function of time.

What has been described above has been intended to be illustrative ofthe invention and non-limiting and it will be understood by personsskilled in the art that other variants and modifications may be madewithout departing from the scope of the invention as defined in theclaims appended hereto. The scope of the claims should not be limited bythe preferred embodiments and examples, but should be given the broadestinterpretation consistent with the description as a whole.

We claim:
 1. A system for monitoring fissile material contents inside ofa nuclear reactor comprising a reactor core containing nuclear fuelmaterial and a radiation shield, the system comprising: a) at least afirst neutron detector positioned outside the radiation shield andconfigured to detect a plurality of neutrons originating from thereactor core and having passed through the radiation shield, andconfigured to generate a first output signal; b) a controllercommunicably linked to the first neutron detector to receive the firstoutput signal and a power output of the nuclear reactor and wherein thecontroller is configured to compare the first output signal to apre-determined, expected neutron flux value per unit reactor power andgenerate an alert if the first output signal differs from the expectedvalue of neutron flux per unit reactor power.
 2. The system of claim 1,further comprising a second neutron detector positioned outside theradiation shield and spaced apart from the first neutron detector, thesecond neutron detector configured to detect the plurality of neutronsoriginating from the reactor core and having passed through theradiation shield, and configured to generate a second output signal,wherein the controller is communicably linked to the second neutrondetector to receive the second output signal.
 3. The system of claim 1,wherein the expected neutron flux comprises at least one of a calculatedneutron flux and an empirically measured base line neutron flux for thereactor.
 4. The system of claim 3, wherein the base line neutron flux isan empirically generated neutron flux value obtained by monitoring thereactor with the system during a calibration session when the reactor isoperating under known conditions and storing the measured neutron fluxin the controller.
 5. The system of any one of claim 2, wherein thefirst neutron detector and second neutron detector comprise large areaneutron detectors.
 6. The system of claim 2, wherein at least one of thefirst neutron detector and the second neutron detector is a fast neutrondetector.
 7. The system of claim 1, further comprising a first moderatorpositioned between the first neutron detector and the radiation shield,the first moderator configured to convert at least some of the pluralityof neutrons from fast neutrons to thermal neutrons before the pluralityof neutrons reaches the first neutron detector.
 8. The system of claim7, wherein the first moderator is formed from high density polyethylene.9. The system of claim 7, wherein the first moderator has a thickness ofbetween 0.5 inches and 3 inches in a direction that the plurality ofneutrons pass through the first moderator.
 10. The system of claim 9,wherein the first moderator has a thickness of 1 inch in a directionthat the plurality of neutrons pass through the first moderator.